POWER GENERATION AND TRANSMISSION 51
Enclosed machines are more fully protected from dust and dampness, but are poorly ventilated, must be larger to avoid excessive heating and cost more.
Motors.—The shunt motor is used for constant speeds, and has high efficiency over a narrow range of speeds and a wide range of loads. Full-load efficiencies vary with size: say from 0.70 to 0.90 for sizes from J£ to 30 hp. Speeds are high for small powers. At a given power, high speed implies lower weight and cost (speed times weight is approximately constant). The cost per pound for motors of 25 hp. is only about half that of 1 hp. motors. The weight per horsepower is around 100 Ib. for all sizes. This is the type of motor used for driving shafting, machine tools, fans, reciprocating pumps and other machines where load variations and starting conditions are not severe and a steady speed is required.
The series motor is used where frequent starts under load are necessary as for traction, and hoisting. Speeds increase as loads (torques) decrease and the motor may "run away" if the load is entirely removed (as by breakage). Power output remains about constant over a wide range of speeds: efficiency is constant for a wide speed range but a narrow power range. Full-load efficiencies vary in about the same way as those for shunt motors, but the maximum is about 3 per cent lower. Five hundred-volt series motors weigh 30 to 60 Ib. per horsepower. Costs per pound are about constant for sizes from 50 to 200 hp.
The compound motor is a compromise type used where frequent starts are necessary and some variation in speed permissible. It has both shunt and series windings, which may act together (cumulative compounding) or oppositely (differential compounding).
Voltages commonly used are 110 to 125 for lighting and small motors, or 220 to 300, for power and lighting; and 500 to 600 for power, particularly traction (for alternating-current voltages see below). The current required by a direct-current motor = (hp. X 746) -j- (efficiency X voltage). Torques of motors otherwise similar in design are proportional to number of poles times current.
Constant speed direct-current motors are usually started by decreasing the amount of an external resistance in the armature circuit. The starting current may exceed full-load current by about 50 per cent. From. 15 to 30 sec. should be allowed to bring a motor, up to speed. This may be insured by automatic operation. The rheostat may incorporate low-voltage and overload releases, and should contain the motor-field circuit closing mechanism. Ordinary rheostats (starters) cost from 3 to 6 per cent as much as motors, the smaller sizes costing relatively more.
Speed Variation.—Speed control of direct-current motors is effected by:
(a) Mul tiple voltage system (three-wire, etc.) Several generators and wires are used, affording a choice of voltages for the armature circuit and therefore of speeds. (Voltage is proportional to r.p.m.) The field voltage is kept constant. Shunt motors are to be used if stability of speed is desired. The efficiency remains good, but the motor is operating at low power output when its speed is low, and the cost of investment is high.
(6) Rheostatic control of armature circuit. This varies voltage and speed, but the speed at any rheostat position varies with the load and the efficiency is low.
(c) Rheostatic control of field circuit. This increases speed by weakening the field. Commutating (inter) poles are desirable if the speed range is to be great. Results are about as with method (a).
To reverse the direction of rotation of a direct-current motor, reverse the terminals to either the field or the armature winding: not to both.
Group vs. Individual Drives.—Individual motor drives are vastly more convenient, but greatly increase installation cost. The effect on every day